Module retry

Convert an HTTP status code to a loop control flag.

Pass this method a numeric HTTP status code.  It returns True if
the code indicates success, None if it indicates temporary
failure, and False otherwise.  You can use this as the
success_check for a RetryLoop.

Implementation details:
* Any 2xx result returns True.
* A select few status codes, or any malformed responses, return None.
  422 Unprocessable Entity is in this category.  This may not meet the
  letter of the HTTP specification, but the Arvados API server will
  use it for various server-side problems like database connection
  errors.
* Everything else returns False.  Note that this includes 1xx and
  3xx status codes.  They don't indicate success, and you can't
  retry those requests verbatim.

Provide a default value for a method's num_retries argument.

This is a decorator for instance and class methods that accept a num_retries argument, with a None default. When the method is called without a value for num_retries, it will be set from the underlying instance or class' num_retries attribute.